Factors affecting choice of structural system
The design should ensure a coordinated approach including structure, envelope, services and finishes.
The principal decisions regarding structure relate to column layout, foundation conditions, integration of building services, and external wall construction.
The design of steel framed buildings encompasses not only the structure, but also the building envelope, services and finishes. All these elements must be coordinated by a firm dimensional discipline which recognises the modular nature of the components to ensure maximum repetition and standardisation in the predetermined grid layout.
The role of a multi-storey building structure is to transmit the applied loads to the foundations.
The principal loads on a multi-storey building are those due to gravity and wind. These are applied at every floor level and on the façade respectively. The structural frame transmits these from their point of application to the foundations. Ideally it should combine structural efficiency with minimum impact on the economy and function of the other elements of the building.
The choice of structural grid defines column positions and is a very important design decision.
The structural grid is defined principally by column positions and the main beams spanning between them. This defines on plan two sets of grid lines which are normally perpendicular. The establishment of this structural grid is a very important design decision and the following points should be considered:
- a column should normally be positioned at every intersection of two grid lines
- a main beam should normally be positioned along every grid line
- ideally grid lines should be orthogonal (the two sets of parallel lines forming a rectangular grid, and the spacing between grid lines should be regular (for circular buildings radial and circumferential grids are often used)
- in practice the shape of the building and/or site may require some variation from this, and irregular spacings or skewed grid lines cannot be avoided. However, these can generally be concentrated in small areas, allowing the main part of the building to be set out in accordance with a regular orthogonal grid.
Where floor voids are required, for example for stair wells or atria, the following guidelines for the location of each void, listed in order of preference, should be followed:
- Locate entirely within a structural bay to avoid cutting across the line of a main beam
- For very large voids, omit complete bays, including main beams if necessary
The omission of a column at a grid intersection has enormous structural implications unless the supporting beams and columns above are also omitted. The exception to this is on the top storey where it may be possible to increase the structural grid by omitting columns on a regular pattern – for example removing alternate columns would effectively double the grid size – and designing the roof structure to span the increased distances. This is generally possible because of the lighter weight construction of roofs compared with floors in normal building construction.
A regular grid improves construction efficiency.
Structural steel floor systems consist of pre-fabricated standard components. If possible columns should be laid out on a repetitive grid which establishes a standard structural bay. This gives maximum repetition of the floor components and thereby reduces fabrication costs and erection time.
Column layout will be influenced by functional requirements.
The function of the building will frequently determine the column layout. For example, financial dealing floors require clear, open spaces located on the lower floors, which would dictate a different structural solution to the rest of the building. Large, column-free areas at ground floor level may necessitate the use of a at first floor to carry the upper floors on an economical column grid.
Typical load transfer systems
The structural grid should coordinate with the planning module and allow for future changes.
Column grids for many office developments have traditionally been in the range 6-9m which coordinates well with the 1.5m planning module. Work patterns are evolving rapidly due to changing commercial practice and the advent of sophisticated electronic communications. A 1.5m module is still common, but longer spans may provide greater future flexibility and can be achieved for only a small increase in cost. A 9m grid is therefore rather more common in current projects, and even longer spans of 12-18m are not unknown.
The rights of light issues of planning considerations may dictate that upper floors are set back from the perimeter, resulting in stepped construction of the upper levels.
Foundations: The design of the superstructure should take account of foundation conditions.
In inner city and difficult sites, the time and cost of constructing the foundations has a major effect on the viability of a project. Although the weight of the frame is relatively small compared with floors and walls, a steel frame can be significantly lighter than a comparable reinforced concrete frame. Further reductions in weight can be achieved by using light floor construction such as composite metal deck floors and lightweight concrete.
Difficult ground conditions may dictate the column grid. Long spans may be required to bridge obstructions in the ground. Generally, widely spaced columns reduce the number of foundations and increase simplicity of construction in the ground.
Widely spread columns on large diameter bored piles
Integration of building services: Service runs can be integrated within the depth of the structure or separated by fixing at a lower level.
The overall depth of the floor construction will depend on the type and distribution of the building services. The coordination of services and structure is an important factor in the choice of an efficient structural floor system. The designer may choose to separate the structural and services zones, or integrate them, allowing for the structural system to occupy the full depth of the floor construction.
Separate service zones result in deeper floors but facilitates future changes.
Separation of zones usually requires confining the ducts, pipes and cables to a horizontal plane below the structure, either resulting in a relatively deep overall floor construction, or close column spacings. However, the services remain reasonably accessible for maintenance and future refitting.
Integration of services and structure reduces construction depth but requires a perforated structure; installation and subsequent refitting may be more difficult.
Integration of services with structure requires either deep perforated structural components, or vertical zoning of the services and structure.
The size and frequency of holes for services will greatly influence the cost of structural floor systems and a highly disciplined approach to design will not only facilitate the construction, but shorten the contract programme and reduce costs.
Building services and floor structure
External wall construction: Cladding costs can be very high and the structure should be designed recognising this and ensuring adequate fixing arrangements.
The external skin of a multi-storey building is supported off the structural frame. In most high quality commercial buildings, the cost of external cladding systems greatly exceeds the cost of the structure. This influences the design and construction of the structural system in the following ways:
- The perimeter structure must provide a satisfactory platform to support the cladding system and be sufficiently rigid to limit deflections of the external wall.
- Reducing the floor zone may be more cost-effective than an overall increase in the area of cladding.
- Fixings to the structure should facilitate rapid erection of cladding panels.
- Reducing the weight of cladding at the expense of cladding cost will not necessarily lead to a lower overall construction cost.